The present disclosure relates to an image-forming apparatus for using heat and pressure to fix paper to which a toner image has been transferred, and to a method for controlling the image-forming apparatus.
In image-forming apparatuses such as printers, multifunctional peripherals, copy machines, and FAX apparatuses, printing is performed by fixing a toner image that has been transferred to paper on the paper. Such image-forming apparatuses are provided with a fixing device that includes a heater and a rotating heating body such as a roller that is heated by the heater and melts the toner. Abnormalities in which the temperature of members such as the roller within the fixing device becomes excessively high may occur as a result of a breakdown in the switch for energizing the heater, a malfunction in the control circuit for controlling the energization of the heater, a crash, or a variety of other causes. The temperature of the fixing device must therefore be prevented from continuing to increase when abnormalities occur.
For example, fixing devices such as that described below are well known. Specifically, fixing devices that are provided with a fixing roller for bearing the downward load from a pressing roller and houses an electric heater, fixing-roller-bearing means that are formed from a thermoplastic resin and hold the fixing roller, a power-source-supplying circuit for supplying a power source to the electric heater, and power-source-supply-blocking means for blocking the power-source-supplying circuit as a result of the fixing roller dropping due to melting of the fixing-roller-bearing means. According to this configuration, in cases where the temperature of the fixing roller has abnormally increased, a rib member melts, and the fixing roller that bears the load from the pressing roller moves in the direction of the load (the downward direction). The connection between the electric heater and the power-source-supplying circuit is blocked in concert with this movement. The need for components and attachments such as thermostats is thereby removed, and costs are reduced.
A fixing device that includes the heater, a rotating heating body for using the heat of the heater to heat the toner image, and a rotating pressing body for pressing against the rotating heating body and applying pressure to the toner image is provided to the image-forming apparatus, as described above. For applying enough pressure the paper, that the rotating pressing body, which is formed from an elastic material such as an elastic resin or a silicone sponge, is pushed against the rotating heating body. The paper to which the toner image has been transferred is passed through the nip portion between the rotating heating body the rotating pressing body. The toner image is thereby heated, pressed, and fixed on the paper.
When the rotating heating body and the rotating pressing body are not made to rotate while energizing the heater, the heat from the rotating heating body concentrates only in those portions of the rotating pressing body that are pressed against the rotating heating body (the temperature increases only in the contacting portions). When this state continues, the temperature of the portions of the rotating pressing body that contact the rotating heating body increases enough that the elastic material begins to melt. When the rotating pressing body begins to melt, a part (the elastic material) of the melted rotating pressing body will adhere to the rotating heating body. When the temperature decreases, the part of the melted rotating pressing body is affixed to the rotating heating body, and the rotating heating body and the rotating pressing body enter a state of integration (referred to as “welding” below). In order to prevent welding, the rotating heating body and the rotating pressing body are made to rotate in high-temperature states in which the heater is energized, and the rotating heating body is heated.
However, the supply of electric power to the image-forming apparatus and the motor that causes the rotating heating body and the rotating pressing body to rotate may suddenly be blocked. The supply of electric power to the image-forming apparatus and the motor may be suddenly blocked in cases where the power cord is pulled out, whether intentionally or as a result of tripping or another accident, cases of pulling-off of the power cords, cases of electric power outage, cases of errors occurring in the circuits related to electrical supply in the image-forming apparatus, or other such cases.
The rotating heating body and the rotating pressing body that were rotating will stop when the electric power supply to the motor is suddenly blocked. Welding may occur when the electric power supply is suddenly blocked in a state in which large amounts of electric power are supplied to the heater for printing or for increasing the temperature of the rotating heating body to a temperature determined in advance (for warming up).
When welding occurs, the rotating heating body and the rotating pressing body will be unable to rotate. Despite the fact that welding prevents rotation, when the motor that causes the rotating heating body and the like to rotate is driven, and drive force is applied, problems may be presented in that the teeth of the gears that transmit the drive force may break off, or other breakage may occur. The motor will be in a locked state, and therefore a large electrical current will flow to the motor for causing the rotating heating body and the rotating pressing body to rotate, temperature increases in the winding and the like in the motor, and the motor may be broken. Beyond the occurrence of welding, problems are thus presented in that the number of malfunctioning sites increases when drive-transmitting portions and the like are broken, and significant repairs are necessary.
The technology according to the aforedescribed fixing device is such that temperature increases in the fixing device are stopped at a set temperature. However, this technology has no bearing on problems of drive force being applied upon the occurrence of welding, whereby gears are broken. The aforedescribed problems therefore cannot be solved. Another aspect of the technology according to the aforedescribed fixing device is that there is no necessary to provide thermostats and temperature fuses. Temperature increases up to the temperature sufficient to melt the rib are therefore permitted. However, when the temperature increases there are concerns not only of welding but of melting of resin members, which include the periphery of the fixing device, and the possibility exists that image-forming apparatus will be irreparably damaged. Therefore, from the perspective of minimizing breakage, the technology according to the aforedescribed fixing device presents many problems.